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first off, i suck at math lol. using the tire size calculator i found my difference in rotation per mile.
Original tires-265/60/16 707.16 revs per mile
new tires- 315/75/16 582.85 revs per mile

so thats a difference of 124.31 revs per mile. so i multiplied 124.31x 5000 (oil change interval, aka miles) which is 621,550. i then devided this by 707.16 (orginal tire revs and what my odometer reads to get 879. so is my math correct? in a 5,000 mile actual distance traveled my odometer would read i only wen 4,121 miles due to bigger tires correct?
 

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Short answer yes. Long answer also yes, see below.

The radius of the original tires will be 14.259" and the radius of the new tires will be 17.3. To calculate the diameter use 2*Pi*Radius which comes out to 89.59" for the original and 108.71" for the new.

Divide 108.71 by 89.59 to come out with a ratio of 1.213. That means that for every 5000 miles the original tire travelled the new one will travel 1.213x5000 = 6066 miles. So if your odometer reads 5000 miles then you are correct that the new tire has travelled only 4,121 miles (5000/1.213).

But as noahvt pointed out the factory tires were actually 265/70/16 so when your odo reads 5000 miles you have actually travelled 4,271.8 miles. Just divide odo by 1.17 and youll be all set.
 

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http://www.tacomaworld.com/forum/tirecalc.php?tires=265-60r16-315-75r16









I would look at my gears as well

2nd gen Xterra differentials & gear ratios

Non-OR 2nd gen with automatic transmissions have the C200 open rear differential, and 3.13 gear ratio.
Non-OR 2nd gen with manual transmissions have the M226 open rear differential, and 3.54 gear ratio.
OR 2nd gen with automatic transmission have the M226 with e-locker rear differential, and 3.36 gear ratio.
OR 2nd gen with manual transmission have the M226 with e-locker rear differential, and 3.69 gear ratio.


EDIT: 3.54 gearing was for ALL MT Xterras in 2005 and early 2006.
Sometime in 2006 (Mayish?) they changed to 3.69 gearing for all MT models. (hat-tip LaRana and XVTer)
 

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Easier (more accurate) way to get revs/mile is to look at the tire's spec sheet from the manufacturer ... it appears you're just using the nominal size (printed on the sidewall), which will likely give you a reasonable answer. Using the published loaded radius (and published revs/mile) gets you that much closer ... at the end of the day, though, the discussed math is still the same.

I'll use my before and after numbers to illustrate:
I started with 265x75r16 BFG Rugged Trails, which have a nominal radius = 15.825", and a nominal revs/mile = 637.231; but the published specs list it as completing 654 revs/mile.
When I went to 255x85r16 Toyo MTs, they should have a nominal radius of 16.533", and a nominal revs/mile of 609.918; and here again, going to the published specs lists the tire as having a revs/mile of 619.

If I use the nominal numbers, the ratio is 1.0448; but if I use the published numbers, then it's actually 1.0565 ... doesn't seem like much; but if you're going to this trouble, then the implication is that you care about that 1.2% difference.

Really though, using your best judgement will be just fine ... if you err on the side of changing oil too early, that's not going to be a bad thing (IMO); might cost a bit more over the life of the vehicle, but if it gives you peace of mind, then go for it.

Actually, after having typed all that out ... the *best* method to figure out your difference, would be to take your GPS, and run a course of 100mi indicated on the odometer ... check it against the GPS reading of how far you've gone, and that'll give you your conversion factor. All of the previous mentioned methods (mine and those discussed earlier in the thread) presume that the odometer is matching up exactly with the 'before' tires ... with the GPS, you're just comparing directly to the odometer reading. The longer 100mi measurement will even out any transient tracking errors or odometer hiccups (that we normally wouldn't otherwise notice).
 

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It's a LOT easier to simply use the old tire dia / new tire dia x desired miles...

example: 31"/33" x 5000 miles = 4696 miles.

As long as you know the correct tire dia for an accurate speedo reading, in our case it's a 32" tire, then using the tire manufacturer's listed dia for a new tire (which will change as the tire wears), you can get accurate numbers. My truck reads about 2 mph slow at 80 mph and I have 33" tires (fairly new). This means the stock speedo is calibrated for the Pro4X model's tires, which are 32" dia. If you're wondering about your speedo calibration, get one of the gps speedo apps and drive on the hwy.

If you're using the stock Xterra speedo calibration, then your particular correction would be:

32/34.6 x 5000 = 4624 miles.
 

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It's a LOT easier to simply use the old tire dia / new tire dia x desired miles...

example: 31"/33" x 5000 miles = 4696 miles.

As long as you know the correct tire dia for an accurate speedo reading, in our case it's a 32" tire, then using the tire manufacturer's listed dia for a new tire (which will change as the tire wears), you can get accurate numbers. My truck reads about 2 mph slow at 80 mph and I have 33" tires (fairly new). This means the stock speedo is calibrated for the Pro4X model's tires, which are 32" dia. If you're wondering about your speedo calibration, get one of the gps speedo apps and drive on the hwy.

If you're using the stock Xterra speedo calibration, then your particular correction would be:

32/34.6 x 5000 = 4624 miles.
I have to disagree with this. I didn't bang out the numbers but diameter and rolling circumference are not proportional. For instance, a tire of diameter 2 has a circumference of 3.14 whereas a tire of diameter 20 has a circumference of 314. That's a factor of 10 difference in the 2 ways of calculating.

I noticed a lot of the earlier threads on tires vs mileage made this error.
 
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